1. Field of the Invention
The invention relates to a speed reduction mechanism that is suitably used in, for example, an electric vehicle that has an electric motor as a driving source and a motor torque transmission device that includes the speed reduction mechanism.
2. Description of Related Art
There is a conventional motor torque transmission device that is mounted in an automobile, and that includes an electric motor and a reduction-transmission mechanism (see, for example, Japanese Patent Application Publication No. 2007-218407 (JP 2007-218407 A)). The electric motor generates motor torque. The reduction-transmission mechanism reduces the speed of rotation output from the electric motor and transmits driving force to a differential mechanism.
The electric motor has a motor shaft that is rotated by electric power from an in-vehicle battery. The motor shaft is arranged along the axis of the reduction-transmission mechanism. Eccentric portions are integrally formed on the outer periphery of the motor shaft. The central axis of each eccentric portion is an axis that is offset by a predetermined eccentric amount from the axis of the motor shaft.
The reduction-transmission mechanism has a pair of reduction-transmission units located around its axis and a housing that accommodates the reduction-transmission units. The reduction-transmission mechanism is interposed between the electric motor and the differential mechanism (differential case), and is coupled to the motor shaft and the differential case. One of the reduction-transmission units is coupled to the motor shaft, and the other one of the reduction-transmission units is coupled to the differential case.
With the above configuration, the motor shaft of the electric motor is rotated by electric power from the in-vehicle battery. The motor torque is transmitted from the electric motor to the differential mechanism via the reduction-transmission mechanism. The motor torque is distributed by the differential mechanism to right and left wheels.
The reduction-transmission units of the motor torque transmission device of this type have a pair of disc-shaped revolving members, a plurality of outer pins and a plurality of inner pins. The revolving members make revolving motions in accordance with the rotation of the motor shaft of the electric motor. The outer pins apply rotation force to the revolving members. The inner pins are arranged radially inward of the outer pins, and output the rotation force of the revolving members to the differential mechanism as driving force (torque), and the driving force is transmitted to a rotation member at wheel side.
The revolving members each have a center hole and a plurality of pin insertion holes. The revolving members are rotatably supported by the eccentric portions of the motor shaft via bearings (cam-side bearings). The central axis of each center hole coincides with the axis of a corresponding one of the eccentric portions of the motor shaft. The pin insertion holes are arranged at equal intervals around the central axis of each center hole.
The outer pins are arranged at equal intervals around the axis of the motor shaft, and are fitted to the housing of the reduction-transmission mechanism.
The inner pins are passed through the pin insertion holes of the revolving members. The inner pins are arranged at equal intervals on a circle around the axis of the rotation member at wheel side, and are fitted to the differential case. Bearings (pin-side bearings) are fitted to the inner pins. The bearings are used to reduce contact resistance between the inner pins and the inner peripheries which define the pin insertion holes of the revolving members.
In the motor torque transmission device described in JP 2007-218407 A, a plurality of outer pins needs to be prepared, and further, the outer peripheral portions of the revolving members need to be formed into a complex shape, which is uneconomical.
To avoid such a problem, external gears may be employed as revolving members, an internal gear may be employed as a rotation force applying member that applies rotation force to the revolving members, and the number of teeth of the internal gear may be set larger than the number of teeth of each of the external gears.
However, if a speed reducer formed of the above-described external gears and internal gear is used in a motor torque transmission device for an automobile, the revolving speed of each of the external gears that are the revolving members becomes relatively high. Accordingly, a load due to centrifugal force acts on the cam-side bearings from the revolving members when the torque is output. As a result, it is necessary to use bearings with high durability as the cam-side bearings, resulting in a cost increase. In addition, because a load due to centrifugal force acts on the cam-side bearings, the service life of each of the cam-side bearings is shortened.